Women being treated with chemotherapy for non-metastatic breast cancer are less likely to survive if they have too much fatty tissue, according to a study conducted by Kaiser Permanente medical scientists.

The issue is not simply too much weight, according to a medical researcher who helped lead the study, but the proportion of fat as opposed to muscle tissue.

Women with “excess adipose (fat) tissue” were significantly less likely to complete their full chemotherapy treatment and, as a result, 30% more likely to die of the disease, the study found.

Elizabeth Cespedes Feliciano, lead author of an article about the study, said that these women were more likely to suffer from the negative effects of the chemo than were women with normal levels of adipose tissue.

They were more likely to drop out of treatment or continue treatment at lower levels of chemotherapy, leading to the lower survival rates, she said.

Feliciano is a research scientist at Kaiser Permanente’s Northern California Division of Research, in Oakland. The article was published last month in the medical journal, JAMA Oncology.

The research found that women who had higher proportions of muscle tissue tolerated the chemotherapy better and were more likely to adhere to and benefit from treatment, Feliciano said.

A key finding, also supported by previous Kaiser research, is that knowledge of body composition – particularly the patient’s proportions of fat and muscle – appears to be essential for determining appropriate chemotherapy dose.

Doctors traditionally base chemotherapy dose on body size, Feliciano said. But the Kaiser research suggests this approach alone can fail to identify details of body composition — which patients have excess adipose tissue —that may leave them more vulnerable to toxic side effects.

Based on earlier Kaiser research, this project started with the supposition that “body composition – not just how big you are but how much muscle and adipose tissue you have and where you are storing them – might influence tolerance for chemotherapy,” Feliciano said.

The study supported the hypothesis. It examined the health and treatment records of 1,395 female patients with non-metastatic breast cancer treated between 2005 and 2013 in Northern California. Non-metastatic means the cancer had not spread beyond the breast.

The women were treated with standard medicines based on the chemotherapeutic agents anthrocycline and taxane. Health records including tomographic scans revealed proportions of fat and muscle tissue and where in the body they were stored.

“We found that patients with greater adiposity were more likely to receive a low dose of chemotherapy,” Feliciano said.

“They were more likely to get the dose reduced. And we also found that those dose reductions, in part, explained the observations that patients with greater adiposity had higher rates of death.”

There are both near-term and long-term lessons from the research, Feliciano believes.

In the long term, it will be worth revisiting the “very simple formula” that chemotherapy dose should be based on body size. “We have better information now” related to body composition, she said.

It will take “years of dose escalation studies and pharmacokinetic studies” to actually change prescribed doses, but in the near term, exercise programs are likely to be helpful for patients with excess fatty tissue.

Exercise is known to be safe and has multiple levels of benefit, including improved morale, higher proportions of muscle tissue and increased tolerance for chemotherapy, she said.

“In chemotherapy…you want to give the maximum dose (the patient) can tolerate. If you give them too much, then they get so ill they can’t continue.

“So exercise… is something ethical and likely beneficial that we can do right now for patients without having to wait for more sophisticated pharmacokinetic studies to come to fruition.”